Ok so, my boyfriend's mom loves making really sweet or fattening dishes and having me try them because none of her kids (3 boys) are interested. And she knows I love food. So anyway, she invited me over for dinner tonight, with my boyfriend of course, his brother, brother's girlfriend, and their aunt. She made spinach lasagne and carrot cake. I know the obvious thing to do is just have a small portion of the lasagne and no carrot cake. But I'm worried about seeming rude, or them asking why I don't want any? Anyway... thought you could all give me some pointers..

btw, Happy November!

if you dont want to say you are on a diet, you could always say you had a big lunch so you're not that hungry. You could also add that you'd love to take some home (of course, you dont necessarily have to eat it when you get home or you can ration it out over a couple of days)

You could say you had a really large lunch, and just have a small sliver of each if you don't want to mention a diet. Or if you feel comfortable you could tell her the truth, that way she won't be offended.
I don't have much advice, i'm married and i still have this problem sometimes with my ils. They don't exactly eat healthy, and get very offended if you don't go back for seconds, or eat desert. Luckily (well, not really) i get sick when i eat foods with a lot of sugar so i have that excuse. As for seconds, i just fill my plate with as much of the salad or vegetable and take small portions of the bad food, that way, with seconds included it is still equal to just one small piece.
Good luck

I usually chug down A LOT of water and just say im full

There are enough people coming to the dinner that you can just take your piece of each item, and then eat a small portion of each -- no one will notice. I promise. If they do, THEN fall back on the big lunch excuse. You can add to them not noticing by volunteering to help clear up the table so you handle your own plate.

I think the keyword here is portion control. You could take some of both (carrot cake may not be the best for you, but as long as you take a little sliver it won't do too much harm). But I'd make a point of only taking a small portion of each, put them on your plate yourself if possible. And of course tell her that it is delicious and that you love it. In that way she can hardly complain, right? If anyone makes remarks about your portions you could say that you are just a small woman and don't need to eat as much as those men .